Britain's choice would be to leave the European Union without a deal or abandon Brexit altogether if its parliament rejects an agreement between the EU and Prime Minister Theresa May, European Council President Donald Tusk said on Friday.

"A few days before the vote in the House of Commons, it is becoming more and more clear that this deal is the best possible -- in fact, the only possible one," Tusk said, ruling out any renegotiation by the EU if the British parliament votes down last Sunday's deal on December 11.

"If this deal is rejected in the Commons, we are left with... an alternative: no deal or no Brexit at all. I want to reassure you that the EU is prepared for every scenario," he said told Reuters during the G20 meeting in Argentina.

Tusk will chair an EU summit two days after the British parliament votes on a deal that May has also said is the best London can get. Britain is due to leave the EU on March 29, although some campaigners hope the withdrawal can be stopped.